1. Field of the Invention
This invention relates to wood burning stoves in which heat from burning wood or similar fuel is used to heat the air in a room or the like. More particularly, the invention relates to such stoves in which hot gases from a fire heat the walls of passageways through which ambient air passes, the ambient air thereby being heated, and in which the flow path of the hot gases increases their contact with the passageway walls to increase the amount of heat transferred to the ambient air.
2. Description of the Prior Art
Wood burning stoves, so named because wood is the principal fuel used with them, have existed for many years. Most such stoves are of generally rectangular box-like shape and are fabricated from iron or steel plate. Common to virtually all wood burning stoves are a fire chamber or fire box in which the wood is placed for burning; draft inlet means to admit air to the fire chamber for combustion of the wood; and a flue or smokestack to allow hot gases and fumes from the fire to escape from the fire chamber. In some stoves the fire chamber includes a hearth area lined with firebrick to support the burning wood; the wood is sometimes placed directly on the brick and sometimes placed on a grate spaced above the brick. Additionally, some such stoves include controls on the draft inlet means which allow the amount of air entering the fire chamber to be decreased to a level which will just support combustion, thereby providing longer burning times for each load of wood burned.
Prior wood burning stoves have been designed in a variety of ways; most early designs of such stoves function as both cooking and heating stoves -- that is, they include flat top surfaces for cooking and/or ovens for baking, and when in use also provide heat by radiation from the hot metal surfaces to the air of the rooms where they are located.
To augment the radiant heat from wood burning stoves, prior workers have devised several methods for providing a flow of heated air from the stoves. In one approach, one or more outer walls are spaced from the fire chamber walls to form an air passageway separate from the fire chamber, the passageway having a lower inlet and an upper outlet for ambient room air. Air in the passageway is heated by contact with the hot fire chamber walls and rises through the outlet, thereby drawing additional air into the passageway. As the additional air is heated, it, too rises; a low volume flow of heated air is thus established to help warm the room. This approach, while useful, is inefficient; i.e., more of the heat from the burning wood escapes through the flue than is extracted to use for heating the room.
Another more recent approach to providing a flow of heated air to a room from a wood burning stove employs what is commonly known as a step stove, in which the stove top is divided into two portions, the rear portion being at a higher level than the front portion, with a short, nearly vertical section of plate joining the two. In this approach, air from the room flows, either by convection or through use of a blower, into a pipe mounted flush with the surface of the hearth floor and extending across the hearth at the front of the fire chamber. Two other pipes, one at each side of the hearth, communicate with the first pipe. Each of the other pipes extends from the front to the rear of the fire chamber, being positioned flush with the surface of the hearth floor; at the rear of the fire chamber each pipe makes a right angle turn and extends vertically upward to a point near the top of the fire chamber, where it makes another right angle turn and extends horizontally toward the front of the fire chamber, finally opening to the room at the short section joining the front and rear portions of the stove top. This stove has improved heating efficiency because the air pipes are within the fire chamber itself and thus extract more heat from the hot gases rising from the fire than is extracted by the previously described passageway arrangement. Even here, however, a substantial amount of the heat from the burning wood passes to the flue; that this is so is evidenced by the fact that the stove described can also heat water circulating through an optional coil which encircles the flue at the rear of the fire chamber.
From the foregoing, it is evident that wood burning stoves of the prior art, while useful to some extent in heating ambient room air, do not efficiently utilize the heat from the burning wood for that purpose. Consequently, more wood is required to heat a given room with prior art stoves than would be necessary with better efficiency.